This application claims the priority of 198 42 624.0-21, filed Sep.17, 1998, the disclosure of which is expressly incorporated by reference herein.
The present invention relates to a method for operating a vehicle steering system which can be operated in a normal mode and in an emergency mode. The system can be switched between a steer-by-wire level assigned to the normal mode, and a fall-back level assigned to the emergency mode. In this arrangement, a steering handle, e.g. steering wheel, which can be actuated by the driver is connected via an electric or electronic control system to steered vehicle wheels. On the fall-back level, the steering handle is positively coupled mechanically and/or hydraulically to the steered vehicle wheels.
In a steering system which can be operated in a steer-by-wire mode or level, the steering handle generally actuates a desired steering-angle transmitter, while the steered vehicle wheels actuate an actual steering-angle transmitter. Such a steering system also has a control unit which carries out a comparison between the desired values and the actual values. As a function thereof, a steering-angle actuator, e.g. a servomotor, is actuated and, in turn, actuates the steered vehicle wheels.
On the steer-by-wire level of the steering system, the steering handle operates without being positively coupled to the steered vehicle wheels. Therefore, it is not possible for the driver to notice side forces acting on the steered vehicle wheels. In particular, the driver cannot tell when the steered vehicle wheels have reached a steering stop with a maximum steering lock.
U.S. Pat. No. 5,347,458 discloses a steer-by-wire steering system for a vehicle, which has a manual-torque actuator which actuates a steering handle in order to simulate at the wheel side forces acting on the steered vehicle wheels. The driver is thereby given steering feedback which corresponds essentially to the steering feedback obtained with a steering system that has positive coupling between the steering handle and the steered vehicle wheels. This improves the "feel" that the driver obtains during driving and the steering stability of the vehicle because the driver receives information on the current driving situation via the side forces that can be felt at the steering handle.
In this context, it is problematic when the steered vehicle wheels are displaced as far as the steering stop since extreme forces or moments at the steering handle must be simulated in order to impart the sense of the steering stop. For example, an electric motor used as a manual-force actuator must be very large to provide the moments required to simulate the steering stop.